The noble metal nanoparticles have excellent catalytic properties, and the porous composite material composed of the noble metal contained in the oxide has many special advantages, such as high thermal stability, large specific surface area, low density, interconnected internal channels and good adsorption and the permeability, other aspects of the sensor, energy storage and conversion, CO catalytic oxidation, and photocatalysis, and have a wide range of applications. The oxide pore walls and noble metal particles can play a better synergy: on the one hand, due to the obstruct by the pore walls of high melting point, the dispersion and thermal stability of the metal nanoparticles are enhanced; on the other hand, the binding surface of the nano-scale oxide and the noble metal can produce a strong interaction effect, which will increase the active points of the composite material and enhance significantly the catalytic performance. Therefore, nano-porous oxide-precious metal composite material have been widely concerned, which are the hot spots of research in recent years.
At present, the methods for preparing the nano-porous oxide-noble metal composite material mainly include the template method and the dealloying method. Using the template method, the porous template is synthesized by chemical means firstly, which is divided into the soft template and the hard template, then the precursor metal ions are infused by impregnation method, the porous template is thermally decomposed by heat treating after drying, to form the porous oxide, and finally the metal particles are loaded in the porous oxide.
Another method for preparing the nano-porous oxide-noble metal composite material is the dealloying method. The dealloying method is essentially the corrosion and decomposition of the alloy, which has become an important method for studying nano-porous metal material at present. The active elements of the alloy containing multiple components are removed during the corrosion, and the remaining inert elements constitute the porous metal structure. The method is simple and suitable for large-scale production. For the preparation of precursor alloys, high temperature melting or electrodeposition, magnetron sputtering and the like can be used, and then it is etched into porous metal structure by the chemical or electrochemical. Recently, the preparation of nano-porous metal composite material containing oxides by the dealloying method has been extensively studied. The oxide element is also retained during the formation of metal porous structure by the design of the appropriate alloy and the use of the corresponding corrosion method. The composite material is formed after heat treatment, and the catalytic activity and thermal stability are significantly enhanced. However, porous composite materials based on rare precious metal are costly and difficult to widely used in some areas. If the low oxide as the matrix, precious metals as the added components, it will be an important development trend which significantly reduces costs under the enhanced catalytic performance.
The dealloying method for preparing the nano-porous oxide includes firstly obtaining porous metal and then oxidizing the porous metal to porous oxide. In another method, some elements do not decompose in the dealloying process, but a part of or all are oxidized to form a porous structure. However, during the the formation of the nano-porous structure, a part of the oxide will be attached to the solid-liquid interface after the formation, preventing the corrosion, while the oxide distribution in the internal is uneven. After the formation of nano-porous oxide and loading it with metal particles, there will be blocking of holes and uneven distribution of metal particles. In addition, in the precursor alloy preparation process, energy consumption is high during the high temperature smelting into the alloy, and the phase structure is complex, or leads to poor pore structure uniformity.
For example, Gao et al. made the dealloying of the melt rapidly quenched Ni—Cu—Mn alloy in ammonium sulfate solution to obtain nano-porous Ni—Mn alloy after the Cu element is dissolved, and then use electrochemical oxidation to form nano-porous oxide-metal composite material. For example again, Ye et al. reported that the element of Al is dissolved during Ti—Mo—Al alloy corroded in the sodium hydroxide solution, and that the remaining elements of Ti and Mo form porous TiO2/MoOx complex. If the alloy contains noble metals, the nano-porous oxide-noble metal composite material will be formed after the dealloying.
However, the above methods have some drawbacks, for example, the template synthesis process is complex, time-consuming, requires inert gas protection or vacuum conditions in the reaction process, is not suitable for large-scale production, and needs a large number of organic solvents, causing some pollution to the environment. In addition, if the noble metal particles are loaded by the impregnated method, this “outside to inside” approach will inevitably lead to a part of the holes being blocked, and thus the specific surface areas will decrease significantly. Moreover, the distribution of metal particles is uneven, and the bonding strength of oxide and metal is relatively weak.